Radio transmitter and receiver antennas are frequently installed at the side of towers, such as telecom and wind turbine towers, and other physical structures, as well as in the vicinity of other systems employing radio transmitters and receivers. Antennas with wide azimuth coverage or that may scan a wide azimuth range may get the physical structure inside its radiation area, where the structure may disturb the antenna function. In addition, antenna arrays often generate a desired main lobe, but also side and back lobes which may reduce the effective gain and directivity of the total array and produce undesired reflections, thereby diminishing the performance of the system.
While the physical structure itself will limit the useable azimuth angle for the antenna, even for azimuth angles outside the physically blocked sector, part of the antenna beam may illuminate the physical structure, reducing accuracy by undesired reflections via the structure, or the structure can produce secondary reflections even when it is not illuminated. Also, the antenna beam must not be pointed such that multipath interference via the structure may disturb the system function. For the antenna to operate at azimuth angles close to the structure, a high gain antenna is required. For an antenna with steered beam, scan angles close to the structure may not be useable. For low gain arrays, the useable scan angle becomes strongly limited due to the wide lobe and possible side lobes. Adding RF absorbing material at the physical structure will reduce the problem. However, as the tower structure may be very large compared to the antenna itself, adding absorber material to the structure itself may be expensive or impractical.
In addition, the proximity of other systems employing radio transmitters and receivers, such as radar and communications system, can limit the usable angle and/or bandwidth of a system. The neighboring radio based systems combined with physical structure interference can severely limit the operational range of antenna-based systems.
Prior art solutions to the problem of obstructions typically involve the use of directional antennas or absorbers. Directional antennas, such as horns, often provide for higher gain, but limit the coverage area of the antenna, thereby requiring more antennas to provide coverage and increasing the cost. The increased number of antennas may also make installation and operation of the antennas more difficult, if the antennas have to be aligned more precisely. The use of absorbers, such as those described in U.S. Pat. No. 5,337,066, reduces the gain of the antenna, which, in turn, typically reduces the coverage distance of the antenna.
Improved antenna solutions are required that overcome the various limitations associated with prior art solutions to enable systems with improved performance and applications.